In  Issue 867536 , the proprietary_codecs flag is being turned on on more trybots and waterfall bots. In this tryjob:
https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win10_chromium_x64_rel_ng/58812

it's found that WebRtcBrowserTest.RunsAudioVideoWebRTCCallInTwoTabsH264 fails. It looks like because these tests run on VMs, no hardware accelerated encoding/decoding is available. It's not clear to me whether this is why the test fails, or whether it's because of the following stack trace which shows up in the logs:

[5312:2372:0726/123820.883:FATAL:thread_restrictions.cc(105)] Check failed: !g_base_sync_primitives_disallowed.Get().Get(). Waiting on a //base sync primitive is not allowed on this thread to prevent jank and deadlock. If waiting on a //base sync primitive is unavoidable, do it within the scope of a ScopedAllowBaseSyncPrimitives. If in a test, use ScopedAllowBaseSyncPrimitivesForTesting.

Backtrace:
	base::debug::StackTrace::StackTrace [0x00007FF6629A7F44+36]
	logging::LogMessage::~LogMessage [0x00007FF6629C4DB2+98]
	base::internal::AssertBaseSyncPrimitivesAllowed [0x00007FF662A2B33B+123]
	base::WaitableEvent::Wait [0x00007FF662A123A2+34]
	content::RTCVideoEncoder::InitEncode [0x00007FF6668576BB+971]
	webrtc::CreateVideoEncoderSoftwareFallbackWrapper [0x00007FF666830883+1283]
	webrtc::VCMGenericEncoder::InitEncode [0x00007FF664694086+374]
	webrtc::VCMEncoderDataBase::SetSendCodec [0x00007FF664695C63+419]
	webrtc::vcm::VideoSender::RegisterSendCodec [0x00007FF6634C5A1D+109]
	webrtc::VideoStreamEncoder::ReconfigureEncoder [0x00007FF66162247D+893]
	webrtc::VideoStreamEncoder::MaybeEncodeVideoFrame [0x00007FF661623008+408]
	webrtc::VideoStreamEncoder::VideoSourceProxy::ResetPixelFpsCount [0x00007FF661625748+680]
	rtc::TaskQueue::Impl::RunTask [0x00007FF66162B51E+126]
	base::internal::Invoker<base::internal::BindState<void (__cdecl rtc::TaskQueue::Impl::*)(std::unique_ptr<rtc::QueuedTask,std::default_delete<rtc::QueuedTask> >) __ptr64,scoped_refptr<rtc::TaskQueue::Impl>,base::internal::PassedWrapper<std::unique_ptr<rtc: [0x00007FF66162BAF1+81]
	base::debug::TaskAnnotator::RunTask [0x00007FF663CD2548+376]
	base::internal::TaskTracker::RunOrSkipTask [0x00007FF663D18245+1173]
	base::internal::TaskTracker::RunAndPopNextTask [0x00007FF663D1748B+507]
	base::internal::SchedulerWorker::RunWorker [0x00007FF665F32100+960]
	base::internal::SchedulerWorker::RunPooledWorker [0x00007FF665F31C20+32]
	base::PlatformThread::GetCurrentThreadPriority [0x00007FF662A277AC+572]
	BaseThreadInitThunk [0x00007FFE73452774+20]
	RtlUserThreadStart [0x00007FFE74020D51+33]

I'm going to try to narrowly disable this test only on this configuration.